Xp=linspace(dx/2-dx,(NI-0.5)*dx-dx,NI);
Yp=linspace(dy/2-dy,(NJ-0.5)*dy-dy,NJ);
dimX = size (Xp,2);
dimY = size (Yp,2);
Hxp = dx * ones (dimX,1);
Hyp = dy * ones (dimY,1);